Patent application title: Multi-phase steel sheet excellent in hole expandability and method of producing the same

Abstract:

The present invention provides a steel sheet excellent in both a balance
between strength and elongation and a balance between strength and hole
expandability, in other words, a multi-phase steel sheet having an
excellent balance between strength and hole expandability.
The present invention is a multi-phase steel sheet excellent in hole
expandability characterized in that:
the steel sheet contains, as chemical components in mass, C: 0.03 to
0.15%, P: not more than 0.010%, S: not more than 0.003%, and either one
or both of Si and Al in a total amount of 0.5 to 4%, and one or more of
Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%, with the balance
consisting of Fe and unavoidable impurities;
the microstructure at a section of the steel sheet is composed of either
one or both of retained austenite and martensite which account(s) for 3
to 30% in total in area percentage and the balance consisting of either
one or both of ferrite and bainite;
the maximum length of the crystal grains in the microstructure is not more
than 10 microns; and
the number of inclusions 20 microns or larger in size at a section of the
steel sheet is not more than 0.3 piece per square millimeter.

Claims:

1-6. (canceled)

7: A method of producing a multi-phase steel sheet excellent in hole
expandability, which steel sheet contains, as chemical components in
mass,C: 0.03 to 0.15%,P: not more than 0.010%,S: not more than 0.003%,
andeither one or both of Si and Al in a total amount of 0.5 to 4%, and
one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%, with
the balance consisting of Fe and unavoidable impurities, characterized
by:when molten steel with said components is refined, circulating the
molten steel not less than 1.5 times after flux for desulfurization is
added at the time of the desulfurization of the molten steel;further,
when a steel sheet is produced by hot-rolling a slab obtained by casting
said molten steel, conducting the finish rolling by controlling the
finish-rolling entry temperature to 950.degree. C. or higher and the
finish-rolling exit temperature within the range from 780 to 920.degree.
C.; andcoiling the steel sheet thus obtained at a temperature of
500.degree. C. or lower.

8: A method of producing a multi-phase steel sheet excellent in hole
expandability according to claim 7, characterized in that the steel sheet
further contains, as chemical components in mass, one or more of Nb, V
and Ti in a total amount of 0.3% or less.

9. A method of producing a multi-phase steel sheet excellent in hole
expandability according to claim 7, characterized in that the steel sheet
further contains, as a chemical component in mass, B of 0.01% or less.

10: A method of producing a multi-phase steel sheet excellent in hole
expandability according to claim 7, characterized in that the steel sheet
further contains, as chemical components in mass, either one or both of
Ca of 0.01% or less and REM of 0.05% or less.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a multi-phase steel sheet excellent
in hole expandability, aiming at the application for automobiles, such as
passenger cars and trucks, etc., for industrial machines, or the like,
and a method of producing the same.

BACKGROUND ART

[0002]In recent years, demands for high strength steel sheets have been
growing with the increasing needs mainly for the weight reduction of
automobile bodies and the assurance of the safety of passengers in a
collision. In particular, the application of steels of TS 590 MPa class
(60 kgf/mm2 class) in tensile strength has rapidly expanded.

[0003]As a steel sheet used for such application, a multi-phase steel
sheet comprising retained austenite and/or martensite is widely known.
For example, as Japanese Unexamined Patent Publication No. H9-104947
discloses, a steel sheet having an excellent balance between strength and
elongation (a total elongation is 33.8 to 40.5% when a tensile strength
is 60 to 69 kgf/mm2) is obtained by containing retained austenite in
an appropriate quantity therein. In this technology, however, a
technology regarding the balance between strength and hole expandability
has not been sufficiently considered and, in particular, technological
requirements for ultra-low P, the control of the maximum length of a
microstructure and inclusions and the control of the hardness of a
microstructure are not, in the least, taken into consideration.
Therefore, the properties of the steel sheet have been inferior (a hole
expansion ratio d/d0 is 1.46 to 1.68, namely 46 to 68% in terms of a net
hole expansion rate, when a tensile strength is 60 to 69 kgf/mm2)
and the application has been limited.

[0004]In the meantime, Japanese Unexamined Patent Publication No.
H3-180426 discloses a bainite sheet steel excellent in the balance
between strength and hole expandability (a hole expansion ratio d/d0 is
1.72 to 2.02, namely 72 to 102% in terms of a net hole expansion rate,
when a tensile strength is 60 to 67 kgf/mm2). However, since this
technology provides not a multi-phase structure but the equalization of a
structure (a bainite single phase structure), as a means of improving the
net hole expansion rate, the balance between strength and elongation is
rather insufficient (a total elongation is 27 to 30% when a tensile
strength is 60 to 67 kgf/mm2) and the application is again limited.

[0005]That is, though, in the press forming of auto parts, punch stretch
formability represented by the balance between strength and elongation
and stretch flange formability represented by the balance between
strength and hole expandability are two major components of forming, such
a technology, satisfying both the components simultaneously, has not been
available and the excellence in both has been the key to the expansion of
the application.

[0006]In recent years while the shift to high strength steel sheets is
progressing at an increasing rate due to global environmental issues, as
their application to components with high degree of forming difficulty
has been taken into consideration, a steel sheet excellent in both the
balance between strength and elongation and the balance between strength
and hole expandability, in other words, a multi-phase steel sheet
excellent in the balance between strength and hole expandability, has
been demanded.

DISCLOSURE OF THE INVENTION

[0007]The object of the present invention is, by solving the problems of
the conventional steel sheets, to provide a steel sheet having both the
excellent balance between strength and hole expandability (not less than
35,000 MPa %, preferably not less than 46,000 MPa %, in terms of the
value obtained by multiplying a tensile strength by a net hole expansion
rate) and the excellent balance between strength and elongation (not less
than 18,500 MPa %, preferably not less than 20,000 MPa %, in terms of the
value obtained by multiplying a tensile strength by a total elongation),
that is, a multi-phase steel sheet excellent in hole expandability, and a
method of producing the same.

[0008]Both of the balance between strength and hole expandability (MPa%),
and the balance between strength and elongation (MPa%) are indexes of
press-formability. If these values are large, the resultant products
exhibit excellent properties. The balance between strength and hole
expandability is represented by the product of the value of strength
(MPa) obtained by tensile test and the value of hole expansion ratio (%)
obtained by hole expansion test. Further, the balance between strength
and elongation is represented by the product of the value strength (MPa)
obtained by tensile test and the value of total elongation obtained by
tensile test. In the steel sheet which is generally used, if tensile
strength increases, both of hole expansion ratio and elongation decrease
and, as a result, both of the balance between strength and hole
expandability (MPa%), and the balance between strength and elongation
(MPa%) exhibit low values. On the other hand, according to the present
invention, lowering the value both of hole expansion ratio and elongation
can be restrained and it is possible to obtain the high values of the
balance between strength and hole expandability (MPa%), and the balance
between strength and elongation (MPa%).

[0009]The present inventors have earnestly studied, from the viewpoint of
integrated manufacturing from steelmaking to hot rolling, and have
finally invented a multi-phase steel sheet excellent in hole
expandability and a method of producing the same.

either one or both of Si and Al in a total amount of 0.5 to 4%, and one or
more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%, with the
balance consisting of Fe and unavoidable impurities;

[0016]the microstructure at a section of the steel sheet is composed of
either one or both of retained austenite and martensite which account(s)
for 3 to 30% in total in area percentage and the balance consisting of
either one or both of ferrite and bainite;

[0017]the maximum length of the crystal grains in the microstructure is
not more than 10 microns; and

[0018]the number of inclusions 20 microns or larger in size at a section
of the steel sheet is not more than 0.3 pieces per square millimeter.

either one or both of Si and Al in a total amount of 0.5 to 4%, and one or
more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%, with the
balance consisting of Fe and unavoidable impurities;

[0024]the microstructure at a section of the steel sheet is composed of
either one or both of retained austenite and martensite which account(s)
for 3 to 30% in total in area percentage, pearlite which accounts for
more than 0% to not more than 3% in area percentage, and the balance
consisting of either one or both of ferrite and bainite;

[0025]the maximum length of the crystal grains in the microstructure is
not more than 10 microns; and

[0026]the number of inclusions 20 microns or larger in size at a section
of the steel sheet is not more than 0.3 pieces per square millimeter.

[0027](3) A multi-phase steel sheet excellent in hole expandability
according to the item (1) or (2), characterized in that the micro Vickers
hardness of bainite is less than 240.

[0028](4) A multi-phase steel sheet excellent in hole expandability
according to any one of the items (1) to (3), characterized by further
containing, as chemical components in mass, one or more of Nb, V and Ti
in a total amount of 0.3% or less.

[0029](5) A multi-phase steel sheet excellent in hole expandability
according to any one of the items (1) to (4), characterized by further
containing, as a chemical component in mass, B of 0.01% or less.

[0030](6) A multi-phase steel sheet excellent in hole expandability
according to any one of the items (1) to (5), characterized by further
containing, as chemical components in mass, either one or both of Ca of
0.01% or less and REM of 0.05% or less.

[0031](7) A method of producing a multi-phase steel sheet excellent in
hole expandability, which steel sheet contains, as chemical components in
mass,

[0032]C: 0.03 to 0.15%,

[0033]P: not more than 0.010%,

[0034]S: not more than 0.003%, and

either one or both of Si and Al in a total amount of 0.5 to 4%, and one or
more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%, with the
balance consisting of Fe and unavoidable impurities, characterized by:

[0035]when molten steel with said components is refined, circulating the
molten steel not less than 1.5 times after flux for desulfurization is
added at the time of the desulfurization of the molten steel;

[0036]further, when a steel sheet is produced by hot-rolling a slab
obtained by casting said molten steel, conducting the finish rolling by
controlling the finish-rolling entry temperature to 950° C. or
higher and the finish-rolling exit temperature within the range from 780
to 920° C.; and

[0037]coiling the steel sheet thus obtained at a temperature of
500° C. or lower.

[0038](8) A method of producing a multi-phase steel sheet excellent in
hole expandability according to the item (7), characterized in that the
steel sheet further contains, as chemical components in mass, one or more
of Nb, V and Ti in a total amount of 0.3% or less.

[0039](9) A method of producing a multi-phase steel sheet excellent in
hole expandability according to the item (7) or (8), characterized in
that the steel sheet further contains, as a chemical component in mass, B
of 0.01% or less.

[0040](10) A method of producing a multi-phase steel sheet excellent in
hole expandability according to any one of the items (7) to (9),
characterized in that the steel sheet further contains, as chemical
components in mass, either one or both of Ca of 0.01% or less and REM of
0.05% or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 is a graph showing the effect of the chemical component P on
a net hole expansion rate.

[0042]FIG. 2 is a graph showing the effect of the maximum length of a
microstructure on a net hole expansion rate.

[0043]FIG. 3 is a graph showing the effect of the number of inclusions on
a net hole expansion rate.

[0044]FIG. 4 is a schematic drawing showing the refining of molten steel
when an RH is used.

[0045]FIG. 5 is a graph showing the effect of the frequency of the reflux
of molten steel after flux addition for desulfurization on the number of
inclusions.

[0046]FIG. 6 is a graph showing the effect of finish-rolling entry and
exit temperatures at the finishing mill in hot rolling on the maximum
length of a microstructure.

BEST MODE FOR CARRYING OUT THE INVENTION

[0047]The present invention is explained in detail hereunder.

[0048]First, the chemical components are explained.

[0049]C is an important element for stabilizing austenite and obtaining a
multi-phase structure, and C is added at not less than 0.03 mass % in
order to stabilize austenite and to obtain either one or both of retained
austenite and martensite in the total amount of not less than 3% in area
percentage. However, the upper limit of C content is set at not more than
0.15 mass %, preferably not more than 0.11 mass %, in order to avoid the
deterioration of weldability and an adverse influence on a net hole
expansion rate.

[0050]P is a key element among the addition elements of the present
invention. The effect of P is demonstrated in FIG. 1. FIG. 1 shows the
result of the investigation on the relationship between the P content and
the net hole expansion rate of a steel sheet, using the steel sheets
having the chemical components of Steel No. 1 in Table 1.

[0051]A net hole expansion rate is calculated based on the Japan Iron and
Steel Federation Standard JFS T 1001-1996. From FIG. 1, the net hole
expansion rate improves remarkably and exponentially by controlling the P
content to not more than 0.010 mass % and its effect on the net hole
expansion rate, which has not yet been assumed within the range of
conventional concepts, is recognized. By so doing, press cracking can be
avoided. Although the reason is not completely clear, it is supposed that
the reduction of P content improves the properties of the edge of a
punched hole (for instance: the minimization of facet size, the reduction
of roughness and the reduction of microcracks on a fractured plane; the
suppression of the deterioration of workability in a microstructure on a
sheared plane; and the like), and leads to the improvement of a net hole
expansion rate.

[0052]S content is set at not more than 0.003 mass %, preferably not more
than 0.001 mass %, from the viewpoint of preventing the deterioration of
a net hole expansion rate and weldability caused by sulfide-system
inclusions.

[0053]Si and Al are elements useful for obtaining a multi-phase structure.
They make either one or both of retained austenite and martensite account
for not less than 3% in total in area percentage and have the function of
improving a net hole expansion rate, by promoting the formation of
ferrite and suppressing the formation of carbide, and further by
strengthening ferrite, thus reducing the hardness difference between
ferrite and hard phases (such as bainite and martensite) and contributing
to the uniformity of a structure. Moreover, they act also as deoxidizing
elements. From the above-mentioned viewpoint, the lower limit of the
total addition amount of either one or both of Si and Al should be not
less than 0.5 mass %. Considering the balance between the cost and the
effect, the upper limit of the total addition amount is set at not more
than 4 mass %.

[0054]With regard to the addition amount of each of Si and Al, the
following may be taken into consideration.

[0055]When excellent surface quality is required in particular, either one
of the means of avoiding Si scale by controlling the Si content to less
than 0.1 mass %, preferably not more than 0.01 mass %, and the means of
making Si scale harmless (making scale less conspicuous by forming the
scale all over the surface) by controlling the Si content rather to more
than 1.0 mass %, preferably more than 1.2 mass %, may be adopted.

[0056]It is also possible to increase the addition amount of Al and reduce
the addition amount of Si to meet the requirement of material properties,
for example, in a case where it is desired to lower a tensile strength by
making use of the difference between Si and Al in the function of
strengthening ferrite.

[0057]Al may be limited to not more than 0.2 mass %, preferably not more
than 0.1 mass %, considering the drawbacks in steelmaking, such as the
erosion of refractory materials, nozzle clogging and the like, and the
material properties.

[0058]Mn, Ni, Cr, Mo, and Cu are elements useful for obtaining a
multi-phase structure, and also are elements which strengthen ferrite.
From the above-mentioned viewpoint, the lower limit of the total addition
amount of one or more of them should be not less than 0.5 mass %.
However, considering the balance between the cost and the effect, the
upper limit of the total addition amount is set at not more than 4 mass
%.

[0059]Furthermore, one or more of Nb, V, Ti, B, Ca and REM may be added as
selective elements.

[0060]Nb, V and Ti are elements effective for a higher strength. However,
considering the balance between the cost and the effect, the total
addition amount of one or more of those elements is set at not more than
0.3 mass %.

[0061]B has a function as a strengthening element, and may be added by not
more than 0.01 mass %. In addition, B also has the effect of mitigating
the adverse effect of P.

[0062]Ca may be added by not more than 0.01 mass % since Ca further
improves a net hole expansion rate by controlling the shape of
sulfide-system inclusions (spheroidizing).

[0063]Moreover, REM may also be added by not more than 0.05 mass % for the
same reason.

[0064]In addition, N may be added by not more than 0.02 mass %, if
required, aiming at the stabilization of austenite and the strengthening
of a steel sheet.

[0065]Next, a microstructure is explained hereunder.

[0066]In order to obtain an excellent net hole expansion rate, from the
viewpoint of not deteriorating the uniformity of a fractured surface
size, one of the properties of the edge of a punched hole, and the like,
which uniformity has been improved by the ultimate reduction of P, the
control of the maximum length of crystal grains in a microstructure and
the control of the amount and size of inclusions are especially
important. Therefore, that is explained first.

[0067]As the crystal grain size of a microstructure affects the fractured
surface size at the edge of a punched hole, it affects a net hole
expansion rate remarkably. Even in the case where the average size of
crystal grains in a microstructure is fine, if the maximum grain size is
large, it adversely affects a net hole expansion rate. As a
microstructure is composed of many crystal grains, a net hole expansion
rate cannot be governed by the average grain size: when a big crystal
grain exists among many crystal grains, it adversely affects the net hole
expansion rate even if the average grain size is fine. Here, with regard
to the size of a crystal grain, not a circle-reduced diameter but the
maximum length thereof affects a net hole expansion rate.

[0068]FIG. 2 shows the result of the investigation on the relationship
between the maximum length of a microstructure in a steel sheet and the
net hole expansion rate of the steel sheet, using the steel sheets having
the chemical components of Steel No. 2 in Table 1. As shown in FIG. 2,
the net hole expansion rate improves remarkably and exponentially when
the maximum length of a microstructure is not larger than 10 microns, and
its effect on the net hole expansion rate, which has not yet been assumed
within the range of the conventional concept, is recognized. By so doing,
press cracking can be avoided.

[0069]Here, the maximum length of a microstructure was calculated from an
optical micrograph under the magnification of 400 taken at a section
perpendicular to the rolling direction of a steel sheet after the section
was etched with a nitral reagent and the reagent disclosed in Japanese
Unexamined Patent Publication No. S59-219473, averaging all over the
section along the thickness direction.

[0070]Moreover, with regard to inclusion control, a net hole expansion
rate can be improved by reducing the number of coarse inclusions. The
number of coarse inclusions was obtained by observing a polish-finished
section along the rolling direction of a steel sheet with a microscope
(400 magnifications) and integrating the number of coarse inclusions 20
microns or larger in maximum length. FIG. 3 shows the result of the
investigation on the relationship between the number of coarse inclusions
(20 microns or larger in maximum length) in a steel sheet and the net
hole expansion rate, using the steel sheets having the chemical
components of Steel No. 2 in Table 1. It is understood that, when the
number of coarse inclusions (20 microns or larger in maximum length) is
not more than a specified number (not more than 0.3 piece per square
millimeter), the net hole expansion rate can be improved remarkably and
press cracking can be avoided.

[0071]In addition, controlling the micro Vickers hardness of bainite to
less than 240 acts preferably on the improvement of hole expandability.
The reduction of the hardness of bainite lowers the hardness difference
between ferrite and bainite and thus contributes to the improvement of
the uniformity of a structure. However, if the micro Vickers hardness of
bainite exceeds 240, the hardness difference between ferrite and bainite
deviates from the range desirable for hole expandability and further the
deterioration of hole expandability is caused by the deterioration of
workability of the bainite itself. The reduction of P (not more than
0.01%) largely contributes to enhancing the effect, but details are not
known.

[0072]Here, the micro Vickers hardness of bainite is obtained by
identifying bainite by etching a section perpendicular to the rolling
direction of a steel sheet with the reagent disclosed in Japanese
Unexamined Patent Publication No. S59-219473, and by averaging the values
measured at five points (averaging the values excluding the maximum and
minimum values from among the values measured at seven points) under a
load of 1 to 10 gr.

[0073]Furthermore, in order to obtain an excellent balance between
strength and elongation as well as an excellent balance between strength
and hole expandability, it is essential to control the kind and the area
percentage of a multi-phase structure.

[0074]An excellent balance between strength and elongation (not less than
18,500 MPa % in terms of the value obtained by multiplying a tensile
strength by a total elongation) and an excellent balance between strength
and hole expandability (not less than 35,000 MPa % in terms of the value
obtained by multiplying a tensile strength by a net hole expansion rate)
are obtained by controlling the total area percentage of either one or
both of retained austenite and martensite to 3 to 30%.

[0075]When the total area percentage of either one or both of retained
austenite and martensite is less than 3%, it becomes impossible to obtain
the stable effect of improving the balance between strength and
elongation, which is to be obtained by the retained austenite and
martensite. Therefore, its lower limit is set at 3%.

[0076]When the total area percentage of either one or both of retained
austenite and martensite is more than 30%, the effect of improving the
balance between strength and elongation is saturated and the
deterioration of a net hole expansion rate and the like are caused.
Therefore, from the viewpoint of press formability, the upper limit of
the total area percentage is set at 30%.

[0077]Here, it is preferable that pearlite is not contained in a steel
sheet since it hinders a balance between strength and elongation and a
balance between strength and hole expandability. Therefore, the area
percentage of pearlite is determined to be not more than 3% at most,
preferably not more than 1%.

[0078]It is more desirable to add the following restrictions in addition
to the above restrictions.

[0079]When a particularly excellent balance between strength and
elongation (not less than 20,000 MPa %) is required, it is desirable that
the area percentage of retained austenite is set at not less than 3%.

[0080]Moreover, when a particularly excellent balance between strength and
hole expandability (not less than 46,000 MPa % in terms of the value
obtained by multiplying a tensile strength by a net hole expansion rate)
is required, it is desirable that the area percentage of martensite is
set at not more than 3%.

[0081]On the other hand, when a low yield ratio (not more than 70% in
terms of yield ratio YR which is a value obtained by dividing a yield
stress by a tensile strength and then multiplying the divided value by
100) is required from the viewpoint of the shape fixability, the area
percentage of martensite is set at not less than 3%.

[0082]Preferably, by controlling the maximum length of the microstructure
of retained austenite and/or martensite to not more than 2 microns, the
effect increases yet further.

[0083]The remainder structure of a microstructure consists of either one
or both of ferrite and bainite, and by controlling the total area
percentage of ferrite and bainite to not less than 80%, the deterioration
of press. formability, which is caused by hard structures other than
ferrite and bainite combining with each other in the form of a network,
can be suppressed.

[0084]Due to the effect described above, both an excellent balance between
strength and hole expandability (not less than 35,000 MPa %, preferably
not less than 46,000 MPa %, in terms of the value obtained by multiplying
a tensile strength by a net hole expansion rate) and an excellent balance
between strength and elongation (not less than 18,500 MPa %, preferably
not less than 20,000 MPa %, in terms of the value obtained by multiplying
a tensile strength by a total elongation) can be obtained simultaneously,
and press formability improves markedly.

[0085]Here, the identification of the constitution of a microstructure,
the measurement of an area percentage, and the measurement of the maximum
length of retained austenite and/or martensite were carried out with an
optical micrograph under the magnification of 1,000 taken at a section
perpendicular to the rolling direction of a steel sheet after the section
was etched with a nitral reagent and the reagent disclosed in Japanese
Unexamined Patent Publication No. S59-219473, and by X-ray analysis.

[0086]Next, the production method is explained hereunder.

[0087]Firstly, when molten steel is refined in a steelmaking process, it
is important to let the molten steel reflux not less than 1.5 times after
the addition of flux for desulfurization at the time when the molten
steel is desulfurized using a secondary refining apparatus such as an RH.
Here, the reflux of molten steel is represented by the amount of molten
steel that circulates the inside of a secondary refining apparatus, such
as an RH, per unit time, and there are various formulas for the
computation. For example, as disclosed in "The Refining Limitation of
Impurity Elements in a Mass Production Scale" (Iron and Steel Institute
of Japan, the Forum of Elevated Temperature Refining Process Section, and
Japan Society for the Promotion of Science, the 19th Steelmaking
Committee, Reaction Process Workshop, March 1996, P. 184-187), the amount
of refluxed molten steel Q expressed by the following Equation 1 is
defined as the refluxed amount of one time:

[0088]The schematic drawing of the refining of molten steel using an RH is
shown in FIG. 4. Two snorkels 3 of the degassing chamber 2 are dipped
into the molten steel ladle 1, gas is blown from underneath one of these
snorkels (in this case, Ar is blown from underneath one of the snorkels
through the injection lance 4), then, the molten steel in the molten
steel ladle 1 rises and enters the degassing chamber 2, and after the
degassing process, the molten steel descends and returns from the other
snorkel 3 to the molten-steel ladle. Here, though the example wherein a
secondary refining apparatus employing an RH is used is shown, it is
needless to say that other apparatus (for example, a DH) may be used.

[0089]FIG. 5 shows the result of investigating the relationship between
the frequency of the reflux of molten steel after flux for
desulfurization is added when molten steel having the components of Steel
No. 2 in Table 1 is refined and the number of inclusions 20 microns or
larger in size per square mm at a section of a steel sheet obtained by
hot-rolling a slab cast from the molten steel. As shown in FIG. 5, by
increasing the frequency of the reflux of molten steel, the surfacing of
the desulfurization flux system inclusions is notably promoted, the
number of coarse inclusions (20 microns or larger) can be reduced to not
more than a prescribed number (not more than 0.3 per square mm), the net
hole expansion rate is improved, and thus press cracking is avoided.

[0090]Next, the condition of the temperature at finish rolling in a
hot-rolling process when a hot-rolled steel sheet according to the
present invention is produced is examined. FIG. 6 shows the result of
summarizing the relation among finish-rolling entry and exit temperatures
when a slab having the components of Steel No. 2 in Table 1 is
hot-rolled, and the maximum length of crystal grains in the
microstructure at a section of the steel sheet obtained.

[0091]As shown in FIG. 6, by regulating the finish-rolling entry
temperature at not lower than 960° C. and the finish-rolling exit
temperature at not lower than 780° C., the maximum length of the
microstructure is certainly controlled to not larger than 10 microns and,
therefore, a net hole expansion rate can be improved and press cracking
can be avoided. Preferably, it is desirable to regulate the
finish-rolling entry temperature in accordance with chemical components,
finish-rolling speed and finish-rolling exit temperature.

[0092]Here, if a finish-rolling exit temperature exceeds 920° C.,
the whole microstructure coarsens, the drawbacks such as the
deterioration of press formability and the generation of scale defects
remarkably appear, and therefore the temperature is determined to be the
upper limit.

[0093]Though conditions on a cooling table after finish rolling are not
particularly specified, the multi-step control of a cooling rate (the
combination of quenching, slow cooling and isothermal retention) or
immediate quenching at the finish-rolling exit, which are generally
known, may be employed, aiming at the control of the area percentage of a
microstructure and the promotion of the fining of a microstructure and
the formation of a multi-phase structure.

[0094]The upper limit of a coiling temperature is set at 500° C. in
order for either one or both of retained austenite and martensite to
account for 3% or more in total in area percentage. If a coiling
temperature exceeds 500° C., the total area percentage of 3% or
more cannot be secured and thus an excellent balance between strength and
elongation (tensile strength multiplied by total elongation) is not
obtained.

[0095]Here, either air cooling or forced cooling may be employed for the
cooling of a steel sheet after it is coiled.

[0096]In addition, a slab may be subjected to rolling after once being
cooled and then reheated, or rolling by HCR or HDR. Further, a slab may
be produced by so-called thin slab continuous casting.

[0097]Furthermore, a steel sheet according to the present invention may be
plated with Zn or the like for improving corrosion resistance, or may be
coated with a lubricant or the like for further improving press
formability.

EXAMPLE

[0098]The chemical compositions other than Fe of the steels subjected to
the test are shown in Table 2.

[0099]The production conditions in the steelmaking and hot rolling of the
steels subjected to the test are shown in Table 3. The microstructures
and material properties of hot-rolled steel sheets obtained are shown in
Tables 4 and 5.

[0103]The maximum length of crystal grains in a microstructure was
calculated from an optical micrograph under the magnification of 400
taken at a section perpendicular to the rolling direction of a steel
sheet after the section was etched with a nitral reagent and the reagent
disclosed in Japanese Unexamined Patent Publication No. S59-219473.

[0104]The number of coarse inclusions in a steel sheet was obtained by
observing a polish-finished section perpendicular to the rolling
direction of a steel sheet with a microscope (400 magnifications) and
integrating the number of coarse inclusions 20 microns or larger in
maximum length.

[0105]The identification of the constitution of a microstructure, the
measurement of an area percentage, and the measurement of the maximum
length of retained austenite and/or martensite were carried out with an
optical micrograph under a magnification of 1,000× taken at a
section perpendicular to the rolling direction of a steel sheet after the
section was etched with a nitral reagent, the reagent disclosed in
Japanese Unexamined Patent Publication No. S59-219473 and the reagent
disclosed in Japanese Unexamined Patent Publication No. H5-163590, and
with X-ray analysis.

[0106]An area percentage of retained austenite (Fγ: in %) was
calculated according to the following equation based on Mo-Kα rays
in X-ray analysis:

where, α(211), γ(220), α(211), and γ(311)
represent the intensity on the respective planes.

[0107]In the examples of the present invention (Nos. 1, 2, 6, 8, 10, 14,
15 and 20), as shown in Table 5, high strength hot-rolled steel sheets
excellent in press formability, having both an excellent balance between
strength and hole expandability (not less than 35,000 MPa % in terms of
the value obtained by multiplying a tensile strength by a net hole
expansion rate) and an excellent balance between strength and elongation
(not less than 18,500 MPa % in terms of the value obtained by multiplying
a tensile strength by a total elongation), are obtained.

[0108]On the other hand, in the comparative examples (Nos. 3 to 5, 7, 9,
11 to 13 and 16 to 19), since some conditions are outside the range of
the present invention as explained at the remarks in Tables 1 to 3, steel
sheets having poor mechanical properties (poor properties in a balance
between strength and hole expandability and a balance between strength
and elongation) are obtained by all means.

[0109]The present invention has made it possible to provide, stably and at
a low cost, a multi-phase steel sheet excellent in press formability,
having both an excellent balance between strength and hole expandability
and an excellent balance between strength and elongation, and a method of
producing the steel sheet, and, consequently, the ranges of the
application and the service conditions have markedly been expanded and
the industrial and economical effects of the present invention are
remarkable.